Platelets play pivotal roles in hemostasis and platelet-related disorders. Activated platelets facilitate recruitment of monocytes and neutrophils to arterial wall and promote atherogenesis. Platelets also initiate the formation of thrombi on ruptured or eroded atherosclerotic plaques, causing clinical complications of atherosclerosis. A striking example occurs in myeloproliferative neoplasms (MPNs) such as essential thrombocytosis and primary myelofibrosis, in which increased platelet production is associated with prominent arterial thrombosis or athero-thrombosis. Aberrant platelet production has also been linked more broadly to cardiovascular risk in the general population. Substantial residual risks of cardiovascular disease in the general population and in patients with MPNs still remain, even with the access of current preventive and therapeutic interventions. HDL may be protective against coronary heart disease. A major hypothesis for the protective role of HDL is that it promotes cholesterol efflux from lesional atheroma cells. Recent studies indicate a key role of ATP-binding cassette transporters, ABCA1 and ABCG1 in promoting cholesterol efflux from macrophage foam cells to HDL. ABCG4 also promotes cholesterol efflux to HDL but ABCG4 is not expressed in macrophages. We found Abcg4 to be selectively expressed in bone marrow megakaryocyte progenitor cells (MkPs). Abcg4-/- MkPs showed defective cholesterol efflux to HDL and increased plasma membrane cholesterol. Abcg4-/- BM transplantation into hypercholesterolemic Ldlr-/- mice resulted in thrombocytosis, accelerated atherosclerosis and arterial thrombosis. Increased platelets reflected an expanded pool of MkPs and megakaryocytes, resulting from increased expression of the thrombopoietin (TPO) receptor (MPL) on the cell surface of MkPs. This reflected blunting of the negative feedback regulation of MPL by the E3 ubiquitin ligase, c-CBL. Further studies suggested that membrane-anchored Lyn kinase is inhibited by association with cholesterol-rich membrane microdomains in Abcg4-/- cells, resulting in impaired activation of c-CBL. We propose that HDL promotes cholesterol efflux from MkPs via ABCG4, activates Lyn and c-CBL, promotes ubiquitination and degradation of MPL and limits MPL-mediated proliferation signaling in response to TPO. HDL inhibits platelet production and suppresses thrombocytosis. We have proposed a series of studies to further test this hypothesis and assess infusion of HDL or pharmacological activation of LYN as potential treatment of atherosclerosis or atherothrombosis caused by aberrantly increased platelet production. Pharmacological activation of Lyn kinase and rHDL infusion will be assessed as potential therapy for MPNs induced by activating mutations of MPL and JAK2. We expect that the proposed studies will provide novel insights and therapeutic strategies for atherosclerosis and atherothrombosis associated with aberrantly increased platelet production.